(CNN) -- A Mexican man who was allegedly killed on orders from his own cartel believed they were hunting for him after he began working as an informant and was fearful for his life, according to court documents.

Police say soldier Michael Jackson Apodaca, 18, acted as the gunman.

Jose Daniel Gonzalez Galeana began to worry after he began working as an informant for immigration officials in the United States.

"The victim was concerned for his own well-being and the safety of his family," the documents said, referencing statements the victim made to a witness.

When Immigration and Customs Enforcement officials gave Gonzalez a visa so he could live in El Paso, Texas, his fellow Juarez cartel members began to get suspicious, El Paso Police Chief Greg Allen said at a press conference.

Allen said Gonzalez's exit from Mexico, combined with a raid on a cartel warehouse and the arrest of cartel lieutenant Pedro "El Tigre" Aranas Sanchez led cartel members to believe he might be working as an informant, Allen said.

Then, a Mexican newspaper named Gonzalez as an informant in the arrest of the high-ranking cartel member, according to court documents. Police say Gonzales quickly became the target of his own cartel.

Police said Gonzalez knew if his fellow cartel members found him, he would likely be killed, police said.

Pfc. Michael Jackson Apodaca, 18, Ruben Rodriguez Dorado, 30, and Christopher Andrew Duran, 17, were each named as suspects Monday and each are facing one count of capital murder. The three men are being held on $1 million bond.

Police said Apodaca was the shooter, Duran was the getaway car driver, and Rodriguez was the one who coordinated the murder.

On Wednesday, police also arrested a 16-year-old who they said was involved in the surveillance and reconnaissance of the victim. The juvenile also faces one count of capital murder.

Rodriguez and the victim were in the Juarez cartel, based across the border from El Paso, police spokesman Chris Mears said. He said police believe Apodaca and Duran were not cartel members.

Apodaca joined the Army a year ago and worked as a crew member on a Patriot missile launcher, officials at Fort Bliss in El Paso said.

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"He was in the top of his class," said Dave Jackson, his grandfather. "You talk to all his sergeants. He's a good soldier. Now, before he went in [and joined the Army], he was in, he was in with a bad crowd."

Investigators said at the press conference they think the three suspects met through a relative of Rodriguez.

"He [Rodriguez] was the older guy, he provided the alcohol, the party locations," police Lt. Alfred Lowe said. "And he recruited these people to do his bidding."

Once the three men were allegedly hired, they were "told to find the victim in the United States," Allen said.

Rodriguez paid the victim's cell phone bill hoping he would be able to learn where he lived, federal officials learned, according to court documents. Federal law enforcement was apparently concerned enough about the incident that officers told local law enforcement to signal Gonzalez's home could be at risk.

Hours before the slaying, police said Rodriguez called a tactical supply store that exclusively sells the same brand of ammunition found at the scene.

The suspects found Gonzalez at a relative's home in Canutillo, Texas, and followed him back to his home before shooting him, police said.

The three arrested men's phone records showed their approximate locations on the night of the killing and corroborated that the suspects were tracking the victim, according to court documents.

During an interrogation, Rodriguez told investigators he ordered Gonzalez's execution, police said. But Allen also said during the press conference that Rodriguez planned the killing and hired the other suspects after he was given the order by cartel lieutenant Jesus Aguayo Salas. Police said there was also a warrant out for his arrest.

Rodriguez told police he paid the participants for their role in the killing, the affidavit said. It said he also noted that he was a midlevel member of the cartel, which he called the "Compania," and said he was in charge of coordinating surveillance for the cartel, "following intended victims up until their execution."

Duran and Rodriguez told police that Apodaca was paid to be the shooter and Duran to drive the getaway car, according to the affidavit. Allen said both men were each paid "quite a robust amount of money ... under $10,000, in that area."